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Full text of "Mathematical And Physical Papers - Iii"

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336          ON  THE  CHANGE  OF  REFRANGIBILITY  OF  LIGHT.
115.    It is needless to say that papers washed with tincture of turmeric, or with a solution  of sulphate of quinine, display their sensibility in a remarkable manner when examined in a linear  spectrum.     The  sensibility  of turmeric  paper  is  rather impaired by .exposing the paper to the light, but on the other hand is materially increased by washing it  with a solution of tartaric acid.
116.    Paper washed  with  an  ethereal  solution  from   dried archil exhibited very well ""the sensibility of that substance.    The derived spectrum consisted chiefly of two distinct portions, one containing orange and a little red, the other consisting chiefly of green, just as in the beam of dispersed light, produced by white light taken as a whole, which the solution itself exhibited. Indeed, I have found that the prismatic composition of dispersed light could be determined even more conveniently by means of a linear spectrum  than  by means  of the  beam   dispersed  by a solution.
117.    The inside of the capsules of the Datura stramonium is nearly white, and apparently uniform.    But when tiie capsules are  examined in  a linear spectrum, certain  patches shine out like bright clouds in the invisible rays.    The whole of the inside is  sensitive,  as  such  substances  almost   always  are, but  these patches, which are probably spots against which the seeds have pressed, are remarkably so.    The capsules were examined after they had begun to burst.
118.    By means of a linear spectrum the sensibility of chlorophyll may be detected in a green leaf.    It is exhibited by the appearance in the derived spectrum of a narrow pure red band of remarkably low  refrangibility.    The  refrangibility  is  so  low that I have always found this band separated from the derived spectrum due to other sensitive substances with which chlorophyll or one of its modifications might have been mixed.
119.    The petals of flowers, so far as I have examined, are as a class rather remarkable for their insensibility, some appearing quite insensible, and others only slightly sensitive.    The bright yellow chaffy involucre of a species of everlasting, proved, however, highly sensitive,  and its sensibility was also  displayed in